The present invention relates to a method for obtaining continuously a particle type polymer by carrying out continuously an emulsion polymerization or a suspension polymerization of a material for polymerization containing a monomer under a flow condition using Taylor vortices and, furthermore, an apparatus to carry out the method.
Polymer particles having a relatively small particle diameter has been used for film formation, elevation of physical properties such as water resistance, acid resistance, and heat resistance, etc., various kinds of controlling agents for viscosity, surface activity, and dispersion, etc. Polymer particles of these kinds need to have a particle diameter distribution as small as possible. This necessity is because of the effectiveness in obtaining an emulsion of relatively high viscosity and also, of the effectiveness in using as a minute spacer and in obtaining a special optical effect (rainbow luminescence etc.).
On the other hand, polymer particles are used having a relatively large particle diameter are used by adding these to a coating or a film as a plastic pigment or as an agent for affording uneven irregularities to a surface. This use is not to obtain a continuous film by using the polymer particles in a molding process, but to use particles that keep their own shape until a final stage.
Although polymer particles having a narrow particle diameter distribution are in many cases desired as the above-described polymer particles, if they are prepared by classifying with sieves or by other kinds of processes, the process becomes complex and also, because the proportion of effective components in products decreases, it is economically unfavorable. Accordingly, a polymerization working process to get polymer particles having a narrow particle diameter distribution has been desired.
To produce polymer particles having a relatively small particle diameter, there has been generally adopted an emulsion polymerization reaction. To produce polymer particles having a relatively large particle diameter, there has been generally adopted a suspension polymerization reaction.
Conventional arts of the emulsion polymerization have been mentioned, for example, in a book entitled as "ENCYCLOPEDIA OF POLYMER SCIENCE AND ENGINEERING", vol. 6, pp. 1 to 51, edited by Jacqueline I. Kroschwitz et al., published by John Wiley & Sons.
The emulsion polymerization is usually a method for producing polymer particles under a condition that an emulsifier, a monomer, and a water-soluble initiator coexists in a water medium, and it has been known as a method for producing small particles of from several ten nanometer to several micrometer.
Conventional arts of the suspension polymerization have been mentioned, for example , in a book entitled as "ENCYCLOPEDIA OF POLYMER SCIENCE AND ENGINEERING", vol . 16, pp. 443 to 473, edited by Jacqueline I. Kroschwitz et al., published by John Wiley & Sons.
The suspension polymerization is a method which comprises adding a hydrophobic monomer in a water medium, dispersing the monomer with stirring, and carrying out a polymerization reaction inside drops of a dispersed solution, and it has been known as a method for preparing particles having a size of a micrometer or larger. As the reaction operation, although it is a polymerization working procedure in a liquid-liquid heterogeneous phase system, an industrially important reaction is in particular a radical polymerization reaction of a monomer having a polymerizable ethylenic carbon-carbon double bond.
It is said that the reaction mechanism of suspension polymerization can be treated in a similar way to a case of bulk polymerization, and that the molecular structure of an obtained polymer is also similar to that of a polymer obtained from the bulk polymerization. Regarding heat transfer, the suspension polymerization differs from the bulk polymerization and solution polymerization and, because the whole system does not convert into a viscous liquid, there is obtained a completely mixing condition, so that there is no serious problem.
A batch method is a preferable main current method for industrially obtaining a polymer by the emulsion polymerization or suspension polymerization. Because, compared with the batch method, a continuous method has difficulty in controlling emulsion conditions or suspension conditions and has a defect of barely getting polymer particles having a desired particle diameter.
However, if the emulsion polymerization or suspension polymerization is carried out by a continuous method, there is an advantage that, compared with the batch method, a large amount of polymer particles can be obtained in a short period of time. Accordingly, if the above defects are overcome, it becomes possible to carry out industrially a continuous polymerization method, so that unmeasurably large benefits are obtained in industry.
There have been proposed, as the continuous polymerization method, several methods such as a method which involves a pipe type reaction vessel, a method which involves a continuous bath type reaction vessel, and a method which involves a loop type reaction vessel, etc. The continuous polymerization method using a pipe type reaction vessel is a method which comprises running continuously a material for polymerization containing a monomer into a pipe having an uniform section and diameter, that is a reaction vessel, and carrying out the emulsion polymerization or suspension polymerization in an interior of the pipe. The continuous polymerization method using a continuous bath type reaction vessel is a method which comprises connecting a number of reaction baths having a stirring means in series, supplying continuously ana successively a material solution into each of the reaction baths, and carrying out the emulsion polymerization or suspension polymerization successively in each of the reaction baths. The continuous polymerization method using a loop type reaction vessel is a method which comprises supplying a material solution into a loop type pipe, circulating the material solution in the pipe to carry out the emulsion polymerization or suspension polymerization, and then, leading the solution to an outside of the pipe.
A problem in conventional arts to be solved is that the particle diameter distribution of a polymer is broad, especially that large particles are very much formed. A reason for this problem is that polymerization reaction time is not uniform by varying with particles (especially, in a case of the emulsion polymerization), or that mixing occurs among liquid drops, that is a phenomenon of dispersion and unification of liquid drops in polymerization. The unification of dispersed liquid drops has been considered as occurring with considerable frequency and, especially, if the polymerization extent becomes 20% or more, the adhesion among particles becomes remarkable and, because of increasing viscosity of drops, redispersion of drops barely occurs and, therefore, there exists a trend of finally gathering together. Thus, an emulsifier or a stabilizer (these are also called as a suspending agent or a protecting agent) is added in order to stabilize the dispersed solution.
Since an unfavorable effect is afforded to the working procedure and quality of products if selection and an amount for use of an emulsifier and a stabilizer are in error, it is desired to keep the amount for use at minimum. However, when the liquid drops are dispersed by violent turbulence such as shear dispersion and turbulence dispersion, collision and reunification of dispersed liquid drops occur with considerable frequency and, therefore, to prevent gathering together of drops and, thereby, to prevent an impossibility of the working, a large amount of the emulsifier and stabilizer must be employed. Accordingly, a working process for a polymerization reaction, with which the collision and reunification of liquid drops are minimized, has been desired.
Although the continuous polymerization method using a pipe type reaction vessel gives polymer particles having a small particle diameter (for example, an average particle diameter of 30 nm), as a ratio of pipe length to pipe diameter (L/D) becomes larger, a product cohering at an inner face of a reaction vessel increasingly generates and, in particular, as the solid portion percentage becomes higher, the cohering product immediately generates and accumulates. Thus, air bubbles easily invade into a material solution for polymerization, or there occurs difficulty in stirring the material solution for polymerization. If the cohering product generates largely, frequent stopping of the reaction apparatus and removal of the product is needed, so that this situation is very fruitless. The cohering product becomes less if the L/D ratio becomes small, but in doing so, the flow of a material solution must be slow in order to secure the reaction time and, as a result, there is not significant difference between this continuous method and a batch method. If air bubbles invade a material solution for polymerization, the cohering product easily generates. Also, stirring of the material solution is necessary in order to accelerate heat transfer and to prevent an one-sided reaction temperature.
In the continuous polymerization method using a continuous bath type reaction vessel, a cohering product on an inner face of the reaction vessel is in a small amount, air bubbles do not invade a material solution for polymerization, and stirring of the material solution for polymerization is easy. However, since the polymerization is carried out by supplying a material solution to each of reaction vessels in sequence, not only polymer particles having a small particle diameter are obtained, but also large polymer particles are obtained simultaneously, so that the particle diameter distribution of obtained polymer particles is broad.
In the continuous polymerization method using a loop type reaction vessel, polymer particles having a small particle diameter can not be obtained, a cohering product easily accommodates at an inner face of the reaction vessel. air bubbles easily invade a material solution, and it is hard to stir the material solution.